Motivated by the challenge of capturing complex hierarchical chemical detail in natural material from a wide range of applications, the Maia detector array and integrated realtime processor have been ...developed to acquire X-ray fluorescence images using X-ray Fluorescence Microscopy (XFM). Maia has been deployed initially at the XFM beamline at the Australian Synchrotron and more recently, demonstrating improvements in energy resolution, at the P06 beamline at Petra III in Germany. Maia captures fine detail in element images beyond 100 M pixels. It combines a large solid-angle annular energy-dispersive 384 detector array, stage encoder and flux counter inputs and dedicated FPGA-based real-time event processor with embedded spectral deconvolution. This enables high definition imaging and enhanced trace element sensitivity to capture complex trace element textures and place them in a detailed spatial context. Maia hardware and software methods provide per pixel correction for dwell, beam flux variation, dead-time and pileup, as well as off-line parallel processing for enhanced throughput. Methods have been developed for real-time display of deconvoluted SXRF element images, depth mapping of rare particles and the acquisition of 3D datasets for fluorescence tomography and XANES imaging using a spectral deconvolution method that tracks beam energy variation.
Nodular chromite is a characteristic feature of ophiolitic podiform chromitite and there has been much debate about how it forms. Nodular chromite from the Troodos ophiolite in Cyprus is unusual in ...that it contains skeletal crystals enclosed within the centres of the nodules and interstitial to them. 3D imaging and electron backscatter diffraction have shown that the skeletal crystals within the nodules are single crystals that are surrounded by a rim of polycrystalline chromite. 3D analysis reveals that the skeletal crystals are partially or completely formed cage or hopper structures elongated along the axis. The rim is composed of a patchwork of chromite grains that are truncated on the outer edge of the rim. The skeletal crystals formed first from a magma supersaturated in chromite and silicate minerals crystallised from melt trapped between the chromite skeletal crystal blades as they grew. The formation of skeletal crystals was followed by a crystallisation event which formed a silicate-poor rim of chromite grains around the skeletal crystals. These crystals show a weak preferred orientation related to the orientation of the core skeletal crystal implying that they formed by nucleation and growth on this core, and did not form by random mechanical aggregation. Patches of equilibrium adcumulate textures within the rim attest to in situ development of such textures. The nodules were subsequently exposed to chromite undersaturated magma resulting in dissolution, recorded by truncated grain boundaries in the rim and a smooth outer surface to the nodule. None of these stages of formation require a turbulent magma. Lastly the nodules impinged on each other causing local deformation at points of contact.
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•Troodos nodular chromite has a skeletal core that in 3D forms a hopper crystal.•EBSD reveals a polycrystalline nodule rim accreted to the core skeletal crystal.•The nodule rim contains adcumulate texture formed by primary crystallisation.•Grains on the outer edge of the nodule rim suffer dissolution and are truncated.•Nodules show evidence of chromite super-saturation followed by chromite under-saturation.
Murrili, the third meteorite recovered by the Desert Fireball Network, is analyzed using mineralogy, oxygen isotopes, bulk chemistry, physical properties, noble gases, and cosmogenic radionuclides. ...The modal mineralogy, bulk chemistry, magnetic susceptibility, physical properties, and oxygen isotopes of Murrili point to it being an H5 ordinary chondrite. It is heterogeneously shocked (S2–S5), depending on the method used to determine it, although Murrili is not obviously brecciated in texture. Cosmogenic radionuclides yield a cosmic ray exposure age of 6–8 Ma, and a pre‐atmospheric meteoroid size of 15–20 cm in radius. Murrili’s fall and subsequent month‐long embedment into the salt lake Kati Thanda significantly altered the whole rock, evident in its Mössbauer spectra, and visual inspection of cut sections. Murrili may have experienced minor, but subsequent, impacts after its formation 4475.3 ± 2.3 Ma, which left it heterogeneously shocked.
Fresh unserpentinised komatiitic dunite at Betheno (Western Australia) contains a distinctive sulfide assemblage of pentlandite, pyrite and millerite. The Ni tenor (i.e. Ni concentration in the ...original sulfide liquid) of this assemblage is in excess of 30 wt%, and the bulk sulfide composition falls within the compositional range of monosulfide solution (MSS) above 800°C. Such assemblages have conventionally been interpreted as the result of hydrothermal upgrading of normal lower Ni-rich magmatic assemblages, but this explanation is not applicable at Betheno. Subtle zonation of Ni concentration in the host olivine, revealed by high-resolution X-ray fluorescence mapping using the Maia detector on the Australian Synchrotron, suggests that coupled subsolidus re-equilibration of Ni and Fe between olivine and sulfide is not a plausible explanation, and the olivine appears to have gained Ni from sulfides rather than the other way around. This leaves a primary magmatic origin as the favoured interpretation, and supports the existence of a stable pyrite-millerite tie-line in the Fe-Ni-S system at low temperatures. Further evidence for this comes from the existence of similar assemblages in fresh dunites from the nearby Perseverance nickel deposit. Hydrothermal alteration is evidently not necessary to form unusually Ni-rich sulfide assemblages. The exceptionally high Ni tenors are attributed to open-system equilibration of sulfide liquid with typical Ni-undepleted olivine, under conditions where sulfide compositions are essentially buffered by the olivine composition, and to the known positive correlation between the Fe/Ni distribution coefficient between olivine and sulfide and the Ni tenor. Other Ni-rich, millerite-bearing assemblages, such as those from the Black Swan nickel deposit, may also have primary origins.
The concentration of platinum-group elements (PGE), Co, Ni, Cu, Re, Au and Ag was determined in base metal sulfides (BMS) minerals and whole rocks of the J-M Reef (footwall, reef and hanging wall) of ...the Stillwater Complex (U.S.A.). The aims of the study were to establish: (i) whether the BMS minerals (pyrrhotite, pentlandite, and chalcopyrite) are the principal host of these elements; (ii) whether these elements preferentially partition into a specific BMS mineral. The results of this study allowed us to consider and evaluate: the possible parental magma composition, the role of sulfide liquid and the role of alteration in the formation of the J-M Reef.
Only a minor quantity of PGE (∼
10 to 140 ppb) were found in the footwall and hanging wall samples of the J-M Reef, which contain only small amounts of sulfur (∼
140 to 400 ppm). In contrast, the reef samples enriched in sulfides contain higher values of PGE (49 to 419 ppm). The S, Ni, Cu, and PGE contents follow the same trends indicating that BMS minerals are the principal phases controlling these elements. Palladium and Pt are the more abundant PGE (up to 244 ppm for Pd and 166 ppm for Pt) with an average Pd/Pt ratio of 3.3. Similarly, the Pd/Ir ratio and the mantle normalized PGE patterns indicate that Pd is more strongly enriched in the reef relative to surrounding rocks than the other PGE.
Pentlandite is the BMS mineral that contains the largest weight fraction of PGE. Palladium represents ∼
95% of the PGE found in solution in BMS minerals and is mainly partitioned in pentlandite. In contrast, Pt is almost exclusively found as platinum-group minerals and do not partition in any BMS minerals. The other PGE are largely found in the BMS minerals, mainly pentlandite.
The variation of S/Se ratios and the presence of secondary magnetite in some samples indicate that these samples may have lost 20 to 50% of their original S. The highest Pd (and by analogy PGE contents) are found in samples containing secondary magnetite, but these are not necessarily those which have experienced the highest S removal. This indicates that the processes which caused the removal of S and the alteration to magnetite are probably distinct.
Our modeling results suggest that PGE (and to a greater extent Pd) enrichment may be summarized into two different steps. First, an immiscible sulfide liquid interacted with a large volume of magma with PGE composition close to high-Mg basalt and collected the PGE. The sulfide liquid percolated down through the crystal mush to collect at a level where the porosity did not permit any further migration. During cooling, magma chamber instabilities triggered the partial desulfurization of the sulfides. Finally, in some parts of reef, a fluid deposited Pd (possibly removed from the footwall) and altered the BMS minerals to magnetite. During this step, Pd was precipitated as an alloy in the most Pd-enriched samples and Pd possibly diffused into pentlandite forming a high-Pd bearing pentlandite. The different contributions of each of the above processes may explain the variability of the Pd grade observed along the J-M Reef.
Major and trace element concentrations in whole-rock and cumulus and intercumulus minerals were determined in cumulate rocks from the Lower and the Lower Critical Zones of the Bushveld Complex, ...South-Africa. These new geochemical data are combined with microtextural observations to provide insights on the formation of the cumulates. The results are used to evaluate methods of calculation of parental liquid composition from which the cumulate rocks crystallized. Cumulus (orthopyroxene and olivine) and intercumulus (clinopyroxene and plagioclase) minerals have a relatively constant composition in terms of major and trace element throughout the Lower and the Lower Critical Zones suggesting that the minerals formed from a magma with a relatively constant composition and followed a similar crystallization history. The minerals are in most cases unzoned in terms of major and trace elements. However, Ti zonations are observed in pyroxene and are consistent with the steady increase in Ti concentrations in interstitial plagioclase with decreasing An. These features are interpreted to be the result of the crystallization of the trapped melt during closed system fractional crystallization without significant intercumulus melt migration as suggested by the absence of dihedral angle modification. Our calculations indicate that the rocks from the Lower and the Lower Critical Zones crystallized from a magma similar in composition to B1-type magma as suggested by previous authors. Our study illustrates that the combined analysis of whole-rock chemistry and cumulus and intercumulus minerals give useful results which allow the estimation of parental liquid composition from cumulate rocks for a wide range of elements. Moreover, our results indicate that the simple assumption of equilibrium between a melt and a mineral to calculate a parental magma can only be applied to pure adcumulate rocks. The methods of calculation presented in this study may be used to infer parental magma composition associated with Ni-Cu-PGE sulfide deposits and can potentially be used as an additional tool for mineral exploration to fingerprint prospective magma suites.
► Determination of trace element concentrations in minerals from the Bushveld Complex. ► Constraints on formation of the cumulates from the Lower and Lower Critical Zones. ► Methods to calculate parental magma composition from cumulus/intercumulus minerals. ► Calculation of the composition of Bushveld parental magma using the proposed method.
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